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Previously, the Service class was responsible for saving and restoring state,
as well as managing file systems. Testing it became problematic.

This patch separates it into two classes Service and Registry, which can
be both tested separately.

TBR=pam@chromium.org
TEST=unit_tests: *FileSystemProvider*Registry*
BUG=248427

Review URL: https://codereview.chromium.org/661393002

Cr-Commit-Position: refs/heads/master@{#300422}